Underwater sand ridges, seen here in radar data from earlier this year, served to "cushion" the blow of the storm, researchers say.

UT Austin Institute for Geophysics

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Originally published on December 12, 2013 7:50 pm

Hurricane Sandy last year did more harm to coastal cities and homes than any hurricane in U.S. history, except Katrina. Most of that damage has been repaired. But there's other damage that people can't see to the underwater coastline, known as the shore face.

Apparently, Long Island's shore face did remarkably well against the storm of the 21st century.

The shore face is the underwater slope that runs up to the shore. Its shape influences how fast and high water moves onto land. Sandy pushed water up the shore face and into towns and bays. When the water retreated, it sucked all sorts of urban junk back out to sea.

Last January, just months after the storm, NPR joined a crew of scientists on an ice-bedecked research boat to survey the damage to the shore. Shivering inside her car, Beth Christensen from Adelphi University showed me a map of which areas got flooded onshore. The map was dotted with hot spots. "This is a power plant," she said, pointing to the map. "This is a sewage treatment plant; here's another sewage treatment; another sewage treatment plant. Here's runoff coming in from the streets through all of those creeks.

"So all of that [material] combined ends up in those sediments," she explained — sediments now lying somewhere offshore.

Onboard the boat that day was John Goff, from the University of Texas, Austin. He was there to scan the seafloor with radar. In addition to finding polluted sediments, Goff wanted to see if parts of the shore face had been washed away; such damage can spell trouble. The shape of the shore face influences how vulnerable the land is to erosion from waves.

"We're going to expect more storms in the future," Goff says, "so understanding the impact of these storms is really important."

The Texas group bills itself as a "rapid response" team — ocean scientists who swoop in after storms to study the damage to shorelines. After Hurricane Ike in Texas in 2008, Goff found that the storm surge had actually remodeled the shore face, moving huge amounts of sand out to sea.

Not so with Sandy. Now, Goff and his team have results.

"What we found was quite different, and yes, it did surprise us," Goff says. The seafloor off Long Island, Goff discovered, has rows of sand ridges — underwater sand dunes up to 10 feet high that run parallel to shore for as far as a half-mile.

"I think of these ridges as kind of cushioning the blow," Goff says. "After the hurricane, they were still there. We didn't really see any massive, destructive erosion of the shore face."

Goff says many coastal areas along the Eastern and Southern U.S. have these underwater sand ridges just offshore. That's going to be important because the sea level is rising. Over time, higher sea levels will eat away at the coastline, undercutting anything that's built there. And during storms, higher sea levels mean it's more likely that wind and waves will push water higher up, onto land. But Goff says these sand ridges seem to slow down erosion and perhaps prevent flooding. That's hopeful news for coastal cities faced with sea level rise.

The news on pollution isn't so good. There were toxic chemicals and metals in the mud at the bottom of estuaries and bays along Long Island. Sandy sucked a lot of that back out into the ocean, where it got spread around. The team will continue to track that.

The researchers reported their findings Friday at a San Francisco meeting of the American Geophysical Union.